The present invention relates to a planar fluorescent lamp with features to provide uniform luminescence.
Thin, planar, durable, easily manufacturable and relatively large area light sources having a range of light intensity are useful in many applications. Such light sources are particularly useful as backlight sources for Liquid Crystal Displays (LCD) to provide readability in a wide range of ambient lighting conditions, from night time to direct sunlight. Such backlight sources are commonly used in avionics, industrial, mobile, and medical applications. Such sources must be very uniform so that the display is not adversely effected by bright or dim spots or patterns of light. Uniform light sources may also be tiled together to form even larger uniform light sources for back illuminating large LCD or transparency images such as X-ray films.
To construct uniform backlight sources the actual light source is placed behind a translucent material that diffuses the underlying light source. The diffuser absorbs light and reduces the amount that reaches the back of the display to be backlighted. The more transparent the diffuser is, the more efficient the backlight system is. However, the more transparent the diffuser is, the less diffusing it does. So, an efficient diffuse backlight system needs to have an inherently uniform original light source so the diffuser can be highly transparent and still be sufficiently diffuse.
Other forms of lighting devices may be used to create uniform light sources. Incandescent lamps and light emitting diodes may be used, however they have poor color qualities and poor luminous efficiency, resulting in high power consumption and heat generation.
An alternative to incandescent lights and light emitting diode arrays is fluorescent technology. Tubular fluorescent lamps have the advantage of being relatively efficient, generating relatively bright light, and having well-established manufacturing capability. In particular, serpentine tubes are especially efficient because they use a long channel length which minimizes parasitic electrode losses. However, tubular fluorescent lamps suffer from fragility requirements when used as optical elements to reflect and diffuse light, are not very durable for harsh environments, and have limited capability to operate effectively and effectively in low light applications.
There are types of flat planar fluorescent lamps that represent improvements in light output uniformity because they incorporate closely spaced patterns of serpentine light. Several planar fluorescent lamps are known in the art such as U.S. Pat. Nos. 3,508,103, 3,646,383 and 3,047,763. Typically such planar lamps are made with two plates of glass spaced apart with a separation material to form channels inside. The channels are coated with phosphorous material, filled with a selected gas and mercury vapor, electrodes are placed at channel ends and a discharge is caused by a high voltage and current flow. In these cases the light is more uniform than separate tubes would create, but there is still non-uniformity between the lighted channels. These kind of planar serpentine lamps do have the added efficiency of long channel lengths described for tubular serpentine lamps.
Lynn et al., U.S. Pat. No. 5,233,262, disclose a planar fluorescent lamp that defines a serpentine channel therein. The light emitted across the planar lamp is non-uniform because of the non-light emitting regions between the channels. To provide increased brightness, increased uniformity, and increased efficiency, an optical reflector is placed in the areas between the lighted channels which directs reflected light toward the front of the lamp. However, in a typical planar lamp the channels are formed into a serpentine pattern. At the turning ends there is an increased unlighted area where the lamp radius diverge. This results in less light per overall area being generated in the serpentine area. As a result, when the entire lamp is covered with a diffuser to homogenize the light, the serpentine bend areas appear less bright and uniformity suffers. It is known in the art that various optical devices, such as Fresnel lens sheets, can be used to concentrate light and to cause linear double imaging, which improves uniformity at the diffuser level, which allows the use of a more transparent diffuser, which makes the backlight system more efficient. However, such films are linear and if they are place in front of a serpentine pattern light source with their elements parallel to the main direction of the light sources, then when the light sources bend in their turns, the film has an undesirable optical result and there is nonuniform light radiated at the diffuser, resulting in non uniform light out of the diffuser. So planar fluorescent lamps that include serpentine lighted patterns in the lighted area of interest require more diffusing to make them uniform and as a result suffer losses in optical efficiency. In applications which require uniform light output across the entire lamp, the curved end portions are masked to prevent the light originating from the curved end portions from reaching the viewer. Unfortunately, the resulting masked display has a reduced useful area, decreased efficiency by having a masked region, a perimeter that is significantly greater than the nonmasked illuminated region, and a manufacturing expense greater than a display having a visible region the same as the nonmasked region.
What is desired, therefore, is a planar fluorescent lamp to create diffused uniform light that has uniform light output, increased efficiency, minimal size, no masking, and reduced manufacturing expense.